In recent years, there has been significant activity in the protection of electrostatically sensitive components from electrostatic discharge during manufacture and transport. Many semi-conductor devices, for example, are electrostatically sensitive and will fail when a low current electrical potential difference is placed across their leads. Such devices have high impedance and the discharge of static electricity causes a breakdown of the gate oxide. Electrostatic potential across the leads of an electronic component may occur directly or indirectly by induction or capacitance charge in the immediate environment.
In order to protect such electrostatically sensitive components during transport and storage, a number of different package arrangements have been previously proposed. In general, these arrangements are designed to either dissipate an electrostatic charge by providing a conductive path between the sensitive component and the ground potential and/or shielding the component from induced potentials in a Faraday cage type construction.
In constructing a Faraday cage type container, an electrostatic shield is configured to completely surround an electrostatically sensitive component to prevent penetration of an induced electrostatic charge. Several types of Faraday cage type containers have been used in the past. One such container is a molded plastic material impregnated with a conductive material such as carbon black. The impregnated carbon black particles form an electrostatic shield which surrounds a sensitive component to be protected. In some instances, plastic has been molded about a wire mesh screen which served as an equipotential surface protecting the enclosed device.
A further Faraday cage type container is disclosed in U.S. Pat. No. 4,241,829 to Hardy. In the Hardy arrangement, a conductive material, such as carbon and graphite, is sprayed or brushed onto the surface of a container to form a continuous equipotential shield for enclosing an electrostatically sensitive component.
A similar but substantially less expensive type of Faraday cage is disclosed in U.S. Pat. Nos. 4,160,503; 4,211,324 and 4,293,070 to Ohlbach. Ohlbach applies a coating of carbon black material onto the exposed exterior surface of a corrugated board through a printing process and thereafter erects a container from the board so that all inside surfaces of the container are coated. When the container is closed, the carbon black coated surfaces completely surround the container contents, protecting any electrostatically sensitive components disposed in the closed container.
Although containers formed with surface coated corrugated panels have been successful, such containers are not without disadvantages. When the carbon black or other conductive material is applied to a surface, the coating is unprotected against physical contact during manufacture and shipment of the panel as well as during the erection and loading of a resulting container. Consequently, the physical integrity of the conductive coating is threatened by abrasion or puncture during all of these processes.
Also, it has become customary in the corrugated container industry to apply graphics and/or decorative coatings to the surface of the containers. The presence of a carbon black conductive coating interferes with and severely limits this practice. Although coating only one side of the panels with conductive coating and erecting a container so that the coating is only on the inside surfaces alleviates the problem of applying graphics to outside surfaces, it further complicates the erection process, since panels have a required orientation and are not reversible.
The application of a carbon black or other conductive coating to only the inside surfaces of a container is advantageous in that it avoids the problems associated with the coating rubbing off during handling and transport. However, coating the inside container surfaces also exposes the packaged article to the coating and increases the possibility of contaminating the packaged article. In order to reduce this possibility, various anti-rub additives have been added to coating formulations. Unfortunately, such anti-rub additives are not totally effective and tend to reduce the ability of resin glues to bond with the corrugated panels during the erection of a container. Furthermore, anti-rub additives further increase the cost of the coating and the resulting container.
The cost of a coated packaging material is also increased when the coating is applied in a printing process, since printing equipment is relatively expensive. Furthermore, when the packaging material includes corrugated board, printing equipment is not easily integrated into conventional machinery. The typical arrangement of a corrugating machine includes components densely packed and laid out in a continuous line. This typical arrangement does not readily afford space to install and operate a printing type applicator. Consequently, prior art methods of applying conductive coatings require a special coating operation separate and distinct from the process of manufacturing the actual corrugated board, thus adding considerably to the cost of conductive coated containers substantially limiting their ready availability.